Measuring Sizes of Ultra-Faint Dwarf Galaxies

TL;DR

This paper investigates how observational choices affect the accuracy of measuring structural parameters of ultra-faint dwarf galaxies, highlighting the importance of survey depth, field-of-view, and star counts for reliable results.

Contribution

It provides simulation-based criteria for accurately determining the sizes of ultra-faint dwarf galaxies in current and future surveys.

Findings

Shallow surveys can lead to up to 100% uncertainties in size estimates.

Field-of-view should be at least three times the galaxy's half-light radius.

More than 1000 stars and a high star density ratio are needed for accurate measurements.

Abstract

The discovery of Ultra-Faint Dwarf (UFD) galaxies in the halo of the Milky Way extends the faint end of the galaxy luminosity function to a few hundred solar luminosities. This extremely low luminosity regime poses a significant challenge for the photometric characterization of these systems. We present a suite of simulations aimed at understanding how different observational choices related to the properties of a low luminosity system impact our ability to determine its true structural parameters such as half-light radius and central surface brightness. We focus on estimating half-light radii (on which mass estimates depend linearly) and find that these numbers can have up to 100% uncertainties when relatively shallow photometric surveys, such as SDSS, are used. Our simulations suggest that to recover structural parameters within 10% or better of their true values: (a) the ratio of the field-of-view to the half-light radius of the satellite must be greater than three, (b) the total number of stars, including background objects should be larger than 1000, and (c) the central to background stellar density ratio must be higher than 20. If one or more of these criteria are not met, the accuracy of the resulting structural parameters can be significantly compromised. In the context of future surveys such as LSST, the latter condition will be closely tied to our ability to remove unresolved background galaxies. Assessing the reliability of measured structural parameters will become increasingly critical as the next generation of deep wide-field surveys detects UFDs beyond the reach of current spectroscopic limits.